In recent years, heat-resistant steel-core aluminum strands (hereinafter referred to as ACSR) have been used for the purpose of increasing power transmission capacity and improving reliability of power systems by one-line operation when there is trouble during the two-line operation. The iron alloy wires incorporated in such heat-resistant ACSR's for field use are generally obtained by coating steel wires of ACSR grade with aluminum or zinc.
Although the Al coating is excellent in resistance to corrosion and heat, it is expensive. The zinc coating improves the resistance of ACSR to corrosion, but to a lesser extent than the Al coating and is inexpensive. It nevertheless forms an Fe-Zn compound and loses toughness on exposure to heat. Further, zinc plating tends to be stripped at high temperatures as described in Nippon Kinzoku Gakkai Shi, 39 (1975), pp. 903-908. Since the temperature at which the ACSR's are used may rise as high as 245.degree. C. at times, the zinc coating has failed to find extensive utility in application to cores of heat-resistant ACSR's.
There also exist zinc alloy coatings. For example, U.S. Pat. No. 4,029,478 describes a Zn-Al alloy coating containing about 0.06-0.15 wt% Pb as an essential component. U.S. Pat. No. 4,056,366 also uses a Zn alloy coating containing about 0.02 wt% Pb and 0.02-0.15 wt% Sb. U.S. Pat. No. 4,152,472 uses a zinc base alloy galvanizing bath containing 0.007-0.10 wt% Pb and 0.005-0.02 wt% Sn as well as Al. The sum of Pb and Sn amounts 0.012 wt% or more.
However, it has now been found that by limiting the amount of Al, Pb and Sb in a Zn-Al alloy coating, unexpectedly superior properties respecting heat resistance can be obtained.